@daneo:
What gauge is appropriate to run from the main battery to the back if 4 is overkill ? Is 8 ok vs 6? Thanks.
Gonna run the numbers here in the spirit of "teach a man to fish", but like others have said, if you want to run 4awg, run 4awg. Especially if you think you'd significantly expand this in the future. What follows will be an engineering-style analysis of what a minimum suggested wire gauge would be for this specific application.
The Jackery 500 DC Input has a 100w maximum according to the product page.
Amazon product ASIN B07SM5HBK1
We'll call that 110w to bake in 10% margin.
The Orion 12/24-20 has an efficiency of 93%. If we ask it for 110 watts on the output, if it's only 93% efficient, we're asking it for 118.27... watts on the input. We'll call it 123 watts to keep the margin going (and to keep the math wicked simple here in a second
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The Orion 12/24-20 has a minimum input voltage of 9 volts. The lowest I've ever seen a (healthy) Toyota temperature compensating alternator go is 12.3 volts. To supply 123 watts at 12.3V, it will take... 10 Amps on the nose! What a coincidence
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We'll use Ohms law to determine how much voltage we can drop between the power source (alternator) and the load (the Orion).
V=IR
V/I = R
(12.3-9) / 10 = allowable resistance
3.3 / 10 = 0.33 ohms
If we take the fuse and mount cold resistance of 0.9mOhms -> 0.0009Ohms. We'll double it for a warm resistance. 0.0018Ohms.
And let's double that resistance again to account for the battery connection and the connection at the Orion. 0.0036Ohms. We'll double that again for the ground connection (which, in reality should be much less if you go straight to the nearest frame bolt for grounding. And there's no fuse inline on the GND side -> 0.0072Ohms.
0.33Ohms is our allowable resistance budget. We subtract out the fuse and connectors resistance of 0.0036Ohms.
0.33Ohms - 0.0072Ohms = 0.3228ohms remaining budget for the wire resistance.
I'll assume a 20 foot run with some snaking around the vehicle and some service loop on each end.
Using the calculator here and a little guess and check...
Wire Resistance Calculator - Cirris Inc - https://cirris.com/wire-resistance-calculator/#quick-calculator
22AWG would be 0.323ohms wire resistance, so over the resistance budget. 20AWG would be 0.203, so well within budget.
There are a couple phenomenon at work here that make 20AWG a feasible cable size:
1) The actual load is very modest. 100 watts.
2) The load is a charger, not a battery. So the load will be regulated and limited to 100 watts, where a "bare" battery in the back could have large, unmitigated peak currents.
3) There's currently a DC-DC converter in the design that allows a much lower working voltage (9V) than a direct battery-to-load connection (12V for the Jackery DC input).
Reasons NOT to run 20AWG wire:
1) Expansion of other loads in the future
2) Probably similar pain-in-the-butt factor to run a higher gauge, like 8AWG, through the firewall gland as 20AWG.
3) Unaccounted for resistances: wire or connectors corrode a bit, etc.
4) Bad data sheets. The data sheets for the Orion and the Jackery leave lots to be desired to do a real engineering analysis, but I did what I could with the consumer-facing data and piling on margin where we could.
I'd probably run 8AWG myself as a balance between plenty of margin for the current design, some expandability in the future, still straight forward to route through the firewall gland & through the scuff plate channel, works well with the fuse crimps, and likely whatever connector you put on the Orion end.
